Photosynthetic Oxygen Exchange

l h e effect of NO,-assimilation on O, exchange and CO, fixation of the cyanobacterium, Synechococcus UTEX 625, was studied mass spectrometrically. Upon addition of 1 mM inorganic carbon to the medium, inorganic carbon pools developed and accelerated O, photoreduction 5-fold when CO, fixation was inhibited. During steady-state photosynthesis at saturating light, O, uptake represented 32% of O, evolution and balanced that portion of O, evolution that could not be accounted for by CO, fixation. Under these conditions, NO,-assimilation reduced O, uptake by 59% but had no influence on CO, fixation. NO,-assimilation decreased both CO, fixation and O, photoreduction at low light and and increased net O, evolution at all light intensities. l h e increase in net O, evolution observed during simultaneous assimilation of carbon and nitrogen over carbon alone was due to a suppression of O, photoreduction by NO,-assimilation. When CO, fixation was precluded, NO,-assimilation inhibited O, photoreduction and stimulated O, evolution. When the electron supply was limiting (low light), competition among O,, CO,, and NO,-for electrons could be observed, but when the electron supply was not limiting (saturating light), O, photoreduction and/or NO,-reduction caused electron transport that was additive to that for maximum CO, fixation.

supporting

confidence: 56%

Inorganic Carbon-Stimulated O2 Photoreduction Is Suppressed by NO2- Assimilation in Air-Grown Cells of Synechococcus UTEX 625

Mir¹,

Salon²,

Canvin³

1995

“…Quenching does not occur in the presence of DCMU which suggests that the quenching is due to oxidation of the acceptor QA of PSII (14). In the presence ofiodoacetamide, CO2 reduction cannot be a means for oxidation of QA (14, 16), but it is well established that 02 can also be reduced by noncycic electron transport (1,4,6 In this communication, we present evidence that an increase in qQ is the major reason for the drop in Chl a fluorescence yield resulting from DIC transport. We also show that DIC transport increases the rate of 02 photoreduction.…”

Section: Introductionmentioning

confidence: 79%

“…Quenching does not occur in the presence of DCMU which suggests that the quenching is due to oxidation of the acceptor QA of PSII (14). In the presence ofiodoacetamide, CO2 reduction cannot be a means for oxidation of QA (14,16), but it is well established that 02 can also be reduced by noncycic electron transport (1,4,6). Addition of 02 can cause Q-quenching (qQ2) even in the absence of CO2 (7).…”

mentioning

confidence: 99%

Active Transport of Inorganic Carbon Increases the Rate of O₂ Photoreduction by the Cyanobacterium Synechococcus UTEX 625

Miller¹,

Espie²,

Canvin³

1988

Chlorophyll a fluorescence ofSynechococcus UTEX 625 was quenched during the transport of inorganic carbon, even when CO2 rfxation was inhibited by iodoacetamide. Measurements with a pulse modulation fluorometer showed that at least 75% of the quenching was due to oxidation of QA, the primary acceptor of photosystem II. Mass spectrometry revealed that transport of inorganic carbon increased the rate of 02 photoreduction. Hence, 02 could serve as an electron acceptor to allow oxidation of QA even in the absence of CO2 fixation.The active transport of HC03-or CO2 causes quenching of Chl a fluorescence in Synechococcus UTEX 625 even when CO2 fixation is inhibited by iodoacetamide (14, 16). Quenching does not occur in the presence of DCMU which suggests that the quenching is due to oxidation of the acceptor QA of PSII (14). In the presence ofiodoacetamide, CO2 reduction cannot be a means for oxidation of QA (14, 16), but it is well established that 02 can also be reduced by noncycic electron transport (1,4,6 In this communication, we present evidence that an increase in qQ is the major reason for the drop in Chl a fluorescence yield resulting from DIC transport. We also show that DIC transport increases the rate of 02 photoreduction. MATERIALS AND METHODSGrowth and Preparation of Cells. Synechococcus UTEX 625 was grown with air bubbling as previously described (5). Cells were washed and resuspended in 25.0 mm BTP/23.5 mM HCI buffer, pH 8.0, which contained low levels (about 15 gM) of DIC (13).Fluorimetry. The fluorescence yield ofChl was monitored with the modulation fluorometer described by Schreiber et al. (21) (PAM-101, H. Walz, D-8521, Effeltrich, FRG). Actinic light to drive DIC uptake and CO2 fixation was from a tungsten-halogen projector lamp with PFR varied with neutral density filters. Fluorescence yield was monitored with a weak (about 2 ME. m2. s_') pulse modulated beam (frequency, I05 s-) and the oxidation state of QA was measured at 20 s intervals with a high intensity white-light flash (1600 uE.m-2.s' for 500 ms). The PAM-101 amplifier system only monitors the fluorescence emission elicited by the modulated light beam, therefore, during the 500 ms saturating flash of actinic light, only changes in fluorescence emission due to changes in the oxidation state ofQA are recorded (21). Fluorescence yield was monitored at the same time as 02 and DIC fluxes were being measured by MS. The Fm was determined in the presence of 20 uM DCMU (14). The Fo was estimated with the modulated light beam at a very low PFR and a frequency of 1.6 x 10-3 s-'. For cyanobacteria, we defined F, which is essentially the same as (F,)m of Schreiber et al. (21), as .Mass Spectrometry. Dissolved 1602, 1802, '2C02, and "3CO2 (m/z = 32, 36, 44, and 45) in the cell suspension (details in figure legends) were sequentially measured at 7-to 15-s intervals using a magnetic sector mass spectrometer (VG Gas Analysis, Middlewich, England, model MM 14-80 SC) equipped with a membrane inlet system (15). Data were stored on magnetic disc fo...

“…'Ordinary' photorespiration presumably occurs on a similar time scale and involves the oxidation of early carbon fixation products, e.g. glycollate (2). Separation of these reactions from the reduction of oxygen by elections from PSI is difficult (25).…”

Section: Resultsmentioning

confidence: 99%

Determination of Oxygen Emission and Uptake in Leaves by Pulsed, Time Resolved Photoacoustics

Mauzerall¹

1990

Pulsed, time resolved photoacoustics has sufficient sensitivity to determine oxygen emission and uptake by single tumover flashes to leaves. The advantage over previous methodologies is that when combined with single turnover flashes the kinetics of the thermal and the gas signals can be resolved to 0.1 millisecond and separated. The S-state oscillations of oxygen formation are readily observed. The gas signal from common spongy leaves such as spinach (Spinacia sp.), Japanese andromeda (Pieris japonica), mock orange (Philadelphus coronarius) and viburnum (Viburnum tomentosum), after correction for instrumental rise time, show a lag of only 1 millisecond and a rise time of 5 milliseconds in the formation of oxygen. Thus a recent proposal that the formation of oxygen requires over 100 milliseconds cannot be true for choroplasts in vivo. The rapid emission is correlated with structure of the leaf. At low light flash energies a rapid gas uptake is observed. The uptake has slightly slower kinetics than oxygen evolution, and its magnitude increases with damage to the leaf. The pulse methodology shows that the uptake begins with the very first flash after dark adaption, and allows the detection of a positive signal (oxygen) on the third flash. These observations, the long wavelength of excitation (695 nanometers) and the magnitude of the signal support the contention that the gas uptake is oxygen reduction by electrons from photosystem 1. These results show that important physiological aspects of a leaf can be studied by pulsed, time resolved photoacoustics.The photoacoustic effect is caused by the thermal expansion of a fluid when the energy of absorbed photons is degraded to heat (17 ever, the inherent complexity of photosynthetic systems coupled with the limited data obtained in the usual photoacoustic methodology using modulated light has made progress slow. A complete analysis would require a dense set of both amplitude and phase measurements over a range of at least three orders of magnitude in frequency. Since the measurements are made one frequency at a time, it is not surprising that such data assets are rare (23). Further, the analysis of such a data set with multiple relaxation times having both positive and negative amplitudes is itself very difficult and may lead to ambiguous results. By contrast, the analysis of the photoacoustic response to a single short pulse of light is relatively straightforward. The signal to noise ratio is typically sufficient to obtain the complete kinetic trace of the signal in a single measurement over the accessible time range of 30 ,us to 100 ms. In addition to this enormous advantage of obtaining all of the accessible time data (i.e. reciprocal frequency) in one measurement, the use of classical single turnover flashes (so short the system can only turnover once [8]) provides the further advantage of identifying and distinguishing processes related to oxygen formation. The slower oxygen pressure signal is easily resolved from the faster photothermal signal because mol...